Certain metals, such as stainless steel that are naturally corrosion resistant may still be susceptible to corrosion or oxidization when the protective oxide layer which isolates the base metal from the oxygen environment is lost. When stainless steel is welded, the Chromium oxide layer in the area of the weld is destroyed. Over time and under proper conditions, the oxide layer grows back, however, the degree of growth depends on the metal surface cleanliness. The welding operation, as well as exposure to free iron particles in the air, or contact with abrasive products, usually leaves deposits that prevent or at least interfere with the growth of the oxide layer.
It is common procedure to clean the weld and remove impurities to promote the growth of the oxide layer and thus help rebuild the passive layer around the weld. However, the cleaning operation is time-consuming, costly and usually involves a chemical treatment such as the application of an acid in combination with an electric current that chemically dissolves the outer metallic layer in the weld area to expose the base metal and promote the regeneration of the oxide layer over a period of typically 24 to 48 hours.
Once the welding and the passivation operation have been completed it is generally impossible to see with the naked eye whether the nanometer-thick oxide layer has properly regrown on the weld and in the adjacent areas. Sometimes, if the passivation operation has not been properly performed, the weld can corrode over time, especially if the material is exposed to a highly corrosive environment in use, or to free iron particles in suspension in the air.
Therefore, given the relatively high costs of stainless steel raw material and transformation processes, there is a need in the industry to determine if a weld or another area of a metal object has been properly passivated and to record the passivation measurements in a file to be able to subsequently demonstrate that all the necessary quality control process steps have been performed during the manufacturing of the metal object.